Abstract Mitochondria are dynamic organelles that function autonomously in many respects to produce energy via glucose (pyruvate) metabolism, undergo morphological change through fusion and fission, and move about the cell. However, these processes are also responsive to signals delivered from the extracellular environment; for example, insulin signals cells to upregulate mitochondrial fusion and alter the production of energy. The regulation of fusion and fission is also key for moving mitochondria properly to synaptic terminals in neurons in response to neurotrophic stimuli. These fundamental processes, when abnormal, cause many types of human disorders including neurodegenerative disease and diabetes. The links between extracellular signaling and mitochondrial responses are understood only in part. We previously uncovered a new role for the signaling lipid Phosphatidic Acid (PA) in mitochondrial fusion [11]. Our more recent unpublished work has connected the production of this signaling lipid on the mitochondrial surface to the generation of an inter-related signaling lipid, Diacylglycerol (DAG). PA can be converted to DAG by the lipid phosphatase Lipin 1, which we have found translocates to mitochondria when surface PA levels increase there. Lipin 1 mutations in mice and humans have been shown to cause a form of lipodystrophy with similarities to Type II diabetes. Taken together, these and other findings suggest that the generation of lipid signals on the surface of the mitochondria may regulate mitochondrial fusion, fission, and energetics in the context of insulin signaling and other extracellular signaling pathways. In this application, we propose in Aim 1 to characterize the external face of the mitochondrial outer membrane as a platform for lipid signaling involving PA and DAG, including analysis of the recruitment of the key enzymes that control their production and elimination, and identification of the physiological signaling pathways that upregulate them. In Aim 2, we will investigate the roles of these signaling lipids in the regulation of mitochondrial fusion, fission, and energy production as a consequence of extracellular signaling. By the end of the proposed experiments, we will have firmly established connections between extracellular agonists, lipid signaling at the mitochondrial surface, and mitochondrial physiological responses in the context of diabetes. Since many of these signaling steps represent drugable targets, gaining insight into the control of these fundamental processes may provide leads to novel therapeutic approaches in diabetes and other disease settings.